Environmental regulations are implemented to restrict the emission of pollutants into the atmosphere. Accordingly, it is necessary to reduce the amount of pollutants to their required level. Many chemical processes, such as fossil fuel conversion processes, release sulfur-containing compounds which must be removed from the effluent stream due to their negative environmental impact and high toxicity to most life forms. Hydrogen sulfide (H.sub.2 S), for example, is one of the most common sulfur-containing compound which needs removal and disposal since this acid gas is extremely toxic and very corrosive in the presence of water. Other toxic sulfur-containing compounds which need to be removed from emitted effluents before entering the atmosphere include: carbon disulfide, sulfur dioxide, carbonyl sulfide, dimethyl sulfide, dimethyl disulfide, methyl mercaptan, and ethyl mercaptan.
In recent years, environmental concerns and the introduction of new environmental laws and regulations have brought about an increasing interest in more effective and economical decontamination processes.
The majority of the biological world derives energy from the oxidation of organic compounds. However, there exist some groups of microorganisms that derive metabolic energy and reducing equivalents for their biosynthesis from the oxidation of inorganic elements and compounds. Biofiltration processes make use of these types of metabolism to remove sulfur-containing compounds from effluent gas streams.
A biofilter is usually composed of an organic material, such as compost, peat or wood bark, having microorganisms attached thereto which are capable of decontaminating an effluent gas stream. The component of interest is degraded when the gas stream passes the biofilter through the metabolizing action of the microorganisms within the filter.
A number of microbial processes for the oxidation of hydrogen sulfide have been described in the patent literature. For example, JP Patent 02126917 describes a deodorizing method. The decontamination is carried out in two peat packing layers carrying microorganisms. Similarly U.S. Pat. No. 5,747,331 discloses a process and an apparatus for the removal of sulfur-containing compounds from waste gases with the aid of microorganisms. The invention described therein is also a two-stage biodegradation process wherein in the second stage the pre-purified gas is contacted again with the microorganisms to degrade any remaining sulfur-containing compounds. The publication in VDI Berichte 735, Biologische Abgasreinigung, VDI Verlag, Duusseldorf, pp. 129-138, 1989, also discloses the use of Thiobacillus spp. microorganisms for the degradation of hydrogen sulfide in waste gases.
Even though the above mentioned processes disclose biofiltration methods for the removal of toxic waste gases, there are still problems associated with the removal strategies, such as short biofilter age or incomplete decontamination.
The design of biofilters depends upon a number of criteria; some of which are the porosity of the filter material, the surface area available for the microorganisms in the organic material to attach to, the loft of the organic material and last but not least, the cost of the media.
It is apparent, that there is a need for effective and lasting biofilters in order to make biofiltration processes more cost effective. The methods disclosed in the prior art attack the problem of biofilter effectiveness by passing the waste gas through two or more biofilters to achieve satisfactory decontamination. Another approach is the post-cleaning or chemical "polishing" of the decontaminated gas stream by passing the purified waste gas through activated carbon. However, the biofilters have to be changed quite frequently due to the deposition of solid residues and physical collapse of the biofilter material. These deposits drastically reduce the capability of the biofilter to remove waste gases since the porosity and the surface area of the biofilter are significantly reduced because of pore blockage. Another aspect of this scenario is the economical disadvantage of the methods described above as the production of biofilters is costly and time consuming.
It is an object of this invention to provide an apparatus and a method for conducting efficient biofiltration processes which are also more cost effective.
It is another object of the invention to provide a method for increasing the lifetime of biofilters.
It is an object of one embodiment of this invention to provide a method and an apparatus for the effective removal of hydrogen sulfide.
The invention disclosed herein obviates this difficulty, by providing a method and an apparatus including two filtration stages with two different filters, a primary stage sacrificial filter and a secondary stage biofilter; a hybrid biofilter system.